Drawing on the Danish Highcrop research projectthis session looked at what we can learn from the Danes using their knowledge in the UK setting. The session covered planning weed control strategies and discussed some of the tools we have available to assist us. This session was organised by Organic Arable
Andrew Trump (Organic Arable): chair.

Bo Melander

Session summary

Andrew Trump introduced the session by highlighting how weeds are a perennial issue for many organic arable farmers. Bo Melander form Aarhus University in Denmark presented results from the HighCrop project focussing on specific management strategies of perennial weeds, particularly common couch grass (Elytrigia repens) and perennial sow thistle (Sonchus arvensis). A combination of strategies are considered including; ensuring flat fields, using competitive crops in the rotation and maintaining soil fertility to feed the crop and starve the weeds. To control severe infestations of couch, Bo advocates employing intensive summer cultivations. This involves harvesting the previous crop in June, repeated cultivations and then sowing a catch crop such as mustard which is then ploughed in during Autumn. He also highlighted the importance of removing couch rhizomes from the field, which can then be composted, and emphasised ploughing, rather than min-till, as essential for perennial weed control in organic systems. By comparing the relative effect of factors such as crop species or cultivations on weed levels over several rotations and sites Bo identified; summer cultivations, adding slurry and winter rye or potatoes as most effective in controlling Couch. Sow thistle is also best controlled by mowing during the ley phase.

Ken Tuffin from Pearce Seeds also shared some findings from initial experimentation with a propane gas burner to control broad leafed weeds in a false seed bed. The machine is less effective against grass weeds as they have a lower growing point in early stages but means that it could possibly be effective in a cereal crop post-emergence without reducing crop yields. He outlined the effectiveness of using the machine at different gas usage rates compared to the economic costs involved per hectare. Ken also considered possible damaging effects on the soil and beneficial insects such as ground beetles. However he found the soil temperature is not drastically raised below about 5cm depth.

William Hudson also described the potential use of the CombCut which works by pulling a series of angled blades and a rotating brush to remove heads of stiff stemmed weeds such as charlock (Sinapis arvensis), thistles and docks (Rumex spp.). This is particularly effective when used in cereal crops before stem extension as the more flexible crop passes beneath the blades undamaged. However, this underlines the importance of timing according to crop growth stage and being able to adjust the angle of the blades.

Key conclusions

The discussion that followed the presentations brought out the following points:

• Effective weed management in organic arable farms relies on an integrated approach combining crop choice in rotations, timely and effective cultivations, fertility management and developing new technologies.
• Bo also described an interesting law against wild oats in Denmark whereby farmers must control the weed below certain levels to avoid the council mowing severely infested parts of the field at the farmer’s expense.